Power amplifiers are widely used in communication systems, for example in radiotelephone base stations and radiotelephones. In radiotelephone communication systems, power amplifiers typically amplify high frequency signals for transmission.
A major consideration in the design of power amplifiers is the efficiency thereof. High efficiency is generally desirable so as to reduce the amount of power that is dissipated as heat. Moreover, in many applications, such as in satellites and portable radiotelephones, the amount of power that is available may be limited. An increase in efficiency in the power amplifier is therefore important, in order to allow an increase the operational time or capacity for the satellite or portable radiotelephone.
A conventional power amplifier such as a class-B amplifier generally only provides maximum efficiency at or near to its maximum saturated power output level. In order to accurately reproduce a signal of varying amplitude, the peak output signal level should be equal to or less than that maximum saturated power level. When the instantaneous signal output level is less than the peak, a conventional class-B power amplifier generally operates at less than maximum efficiency.
The efficiency generally reduces as the square root of the output power. This is because, using the class-B example, the output power reduces as the square of the output current but the power consumption from the battery or other DC supply reduces only proportional to the output current. Therefore, the efficiency, which is the ratio of output power to battery power, reduces proportional to the current, i.e., proportional to the square root of the output power.
Accordingly, a power amplifier that has 60% efficiency at a peak output of 2 watts will generally have no more than 42% efficiency at an output of 1 watt (3 dB reduced output). Moreover, when amplifying a signal of varying amplitude, a conventional amplifier may not produce an output signal amplitude proportional to the input signal amplitude, thereby causing nonlinear distortion and intermodulation.
With a varying output signal power P(t)=A.sup.2 (t), the average efficiency can be estimated to be: ##EQU1##
Nonlinearities in conventional amplifiers can be reduced by various techniques, such as by an inverse predistortion of the input signal, or by feedback including Cartesian feedback in radio frequency power amplifiers for linearly amplifying signals with a bandwidth much less than the center frequency. Unfortunately, linearization generally does not alter the above efficiency formula, which in fact already assumes that the output amplitude can be made to faithfully follow the desired varying amplitude waveform. In effect, the average efficiency calculated above already assumes perfect linearization.
The loss of efficiency comes about because current I(t) is drawn from the battery at a constant voltage Vcc, but is supplied to the load at a varying voltage I(t).multidot.RL which is less than Vcc. The voltage difference Vcc-I(t).multidot.RL is lost across the output device (e.g. collector junction), causing power dissipation in the device.
In U.S. Pat. No. 2,210,028 to Doherty (Aug.1940), an arrangement of two vacuum tube power amplifiers coupled by a single quarter-wave line is described. The first amplifier is operated up to an output level of Pmax/4, at which it achieves maximum practical class-B efficiency. For powers above this level, the second amplifier is caused to contribute. The second amplifier affects the load impedance of the first amplifier one quarter wave away such that the first amplifier can increase its power up to Pmax/2, while the second amplifier also contributes up to Pmax/2, making Pmax in total, at which point both amplifiers are once more achieving maximum practical class-B efficiency. Thus, efficiency is preserved over a 6 dB range of output levels from Pmax/4 to Pmax. A semiconductor version of the Doherty amplifier is described in a more recent U.S. Pat. No. 5,420,541 entitled "Microwave Doherty Amplifier" to Upton et al.
In the prior art Doherty amplifier, the "normal" power amplifier amplifies a signal from 0 power to 1/4 the peak power level, achieving maximum class-B efficiency at that power level. The peak power amplifier then begins to contribute to the output power and by reducing the effective load impedance seen by the "normal" power amplifier, enables it to generate a greater power output up to half the peak power level. The peak power amplifier also generates half the peak power level so that the two amplifiers jointly produce the desired peak power level. The "peak" power amplifier in this prior art is not operated in antiphase so as to detract from the output power level, and thereby increasing the effective load impedance seen by the "normal" power amplifier and allowing it to generate less power efficiently. Thus the "peak" power amplifier does not operate symmetrically as a "trough" power amplifier.
In Proc. IRE, Vol. 23 No. 11 (1935), pages 1370-1392, entitled "High Power Outphasing Modulation", Chireix describes producing a transmitter giving a modulated amplitude output signal by combining two constant output amplitude amplifiers with a variable phase difference so that their outputs can be varied in relative phase from additive to subtractive. The Chireix and Doherty techniques were not combined to obtain an amplifier of good linearity and high efficiency, as the Doherty amplifier relied on the two constituent amplifiers being co-phased while the Chireix amplifier relied upon them being out-of-phase. When two amplifiers are out-of-phase, as they were in the prior art, they are preferably isolated from one another using a hybrid coupler or directional coupler to combine them. The directional coupler combines the two amplifier's output signals to produce a sum signal and a difference signal, the sum signal being used as the desired output and the difference signal being terminated in a dummy load. Since all the amplifier power ends up at either the sum or the difference port and is not reflected to either amplifier, the amplifiers are isolated from one another and do not affect each other's load line.
In U.S. Pat Nos. 5,568,088; 5,574,967; 5,631,604; and 5,638,024 to applicant Dent, all entitled "Waste Energy Control and Management in Power Amplifiers ", various arrangements of coupled power amplifiers are disclosed in which a varying amplitude signal may be produced using constant amplitude power amplifiers. In one arrangement, two constant power amplifiers are driven with a relative phase shift as in Chireix such that their outputs add more or less constructively or destructively to produce a varying output. The amplifiers were coupled at their outputs using a hybrid coupler or directional coupler which forms both a sum signal and a difference signal. An improvement over the prior art described therein comprises recovering the normally wasted energy at the difference port using a rectifier circuit. The Doherty patent, the Chireix paper and the above referenced Dent patents are hereby incorporated by reference herein.
In applicant's 1964 graduate thesis project, an amplifier was built and reported in which the value of Vcc was selected to be either Vcc or 0.7 Vcc based on whether the desired output amplitude was greater or less than 0.7 Vcc. With a pure sine wave drive, this raised the peak efficiency from the theoretical value of .pi./4 (.about.78.5%) for a class-B amplifier to 85.6% for the new amplifier, termed class-BC. The efficiency at half maximum output power was now 78.5% instead of 55% for class-B.
The Vcc selection was effected by using a first pair of transistors connected to the 0.7 Vcc supply to supply load current when the output amplitude was less than 0.7 Vcc, and a second pair of transistors connected to the full Vcc supply for supplying the load current for amplitudes between 0.7 Vcc and Vcc. Diodes were used to protect the first pair of transistors by preventing reverse current flow when the output amplitude was driven above their supply voltage. The above arrangement worked well for audio frequencies where diodes turn on and off sufficiently fast, but may not be effective for microwave frequencies.
Also in the 1960s, many so called "class-D" or pulse-width modulation amplifiers were proposed and manufactured. Pulse-width modulation amplifiers switched the output devices on and off at a high frequency with a mark-space ratio proportional to the instantaneous desired signal waveform. A low-pass output filter smoothed the switching signal to reject the high switching frequency and to produce the mean of the varying mark-space ratio signal as the desired output signal waveform. A disadvantage of the class-D amplifier was the need to switch the output devices at a very much higher frequency than the desired signal to be amplified, which may not be practical when the desired signal is already a high frequency signal such as a microwave signal.
The above survey indicates that many techniques have been used in order to improve the efficiency of power amplifiers. However, notwithstanding these techniques, there continues to be a need for power amplifiers that can operate at high efficiencies at maximum output, and also at outputs that are below maximum output. Moreover, it is desirable for high efficiency power amplifiers to operate with high frequency signals, such as are used in wireless communication systems.
Parent application Ser. No. 09/054,063 describes the coupling two amplifiers that are driven using Chireix outphasing modulation to one another, so that the amplifiers affect each other's effective load line. The two amplifiers can thereby maintain efficiency over a wider dynamic range than in a conventional Doherty amplifier.
More specifically, the parent application provides apparatus that amplifies an AC input signal of varying amplitude and varying phase using a DC power supply. The apparatus includes a converter that converts the AC input signal into a first signal having constant amplitude and a first phase angle and into a second signal having constant amplitude and a second phase angle. A first amplifier amplifies the first signal, and a second amplifier amplifies the second signal. A coupler couples the first and second amplifiers to one another and to a load impedance, such that voltages or currents in the first amplifier become linearly related to voltages or currents in the second amplifier.
In one embodiment, the coupler comprises at least one transformer that serially couples the first and second amplifiers to one another and to the load impedance. In another embodiment, the coupler comprises first and second quarter wave transmission lines that couple the respective first and second amplifiers to one another and to the load impedance.
According to another aspect of the parent application, the first and second amplifiers are first and second bilateral amplifiers, such that current flows from the first and second amplifiers to the DC power supply during part of the signal cycle of the AC input signal, to thereby return energy to the DC power supply. Further increases in efficiency may thereby be obtained.
Accordingly, two coupled amplifiers driven using the outphasing modulation of Chireix can operate identically and can symmetrically affect each other's effective load line so as to efficiently generate both peak and trough power levels and maintain efficiency over a wider dynamic range than in a Doherty amplifier. When the two amplifiers that are not in phase affect each other's load line, current flows from the DC source to the load during part of the signal waveform cycle and flows to the source for another part of the cycle. The mean power consumption from the source can be reduced in the same ratio as the load power is reduced, thus maintaining efficiency. In the Chireix and Doherty disclosures, vacuum tubes of that era were not able to conduct in the reverse direction to return current to the source. In contrast, in the present invention, two amplifiers constructed using bilateral devices are driven by two, separate, preferably digitally synthesized waveforms and their outputs are combined, for example using transformers or two quarter wave lines connected to a harmonic short circuit. Using the parent application, the linearity advantage of Chireix may be obtained together with an even greater efficiency improvement than Doherty's technique.
When two constant amplitude signals are to be combined to produce a varying amplitude, the phase of each signal is varied in the opposite direction to one another. When the phase of the resultant also varied, the desired phase variation is added to the phases of each signal. The direction of phase variation is additive to the amplitudes-varying phase component in the case of one of the signals, and subtractive from the other. Thus, the phase of one signal may need to vary more rapidly.
When a phase locked loop is used to produce the desired varying phases at a desired radio frequency, the loop bandwidth that is needed to follow the more rapidly varying phase therefore may need to be increased. This increase in phase locked loop may allow more undesirable noise amplification.